X-ray diagnostic apparatus

ABSTRACT

According to one embodiment, an image generation unit generates a first image during a large aperture period and a second image during a small aperture period. An image combining unit generates a composite image based on the latest second image and the specific first image. A display unit displays the composite image in real time. A determination unit determines whether to update the first image based on an index associated with the anatomical positional shift between the first image and the second image. A driving control unit enlarges a aperture to the large aperture, when the determination unit determines to update, and maintains the aperture at the small aperture, when the determination unit determines not to update.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT Application No.PCT/JP2012/072203, filed Aug. 31, 2012 and based upon and claiming thebenefit of priority from Japanese Patent Application No. 2011-223305,filed Oct. 7, 2011, the entire contents of all of which are incorporatedherein by reference.

FIELD

Embodiments described herein relate generally to an X-ray diagnosticapparatus.

BACKGROUND

In an ablation procedure, an X-ray diagnostic apparatus is used to bringa catheter, a guide wire, or the like to a treatment region such as aheart. This ablation procedure often continues for a long period andoften takes several hours. Therefore, there have been developedtechniques for reducing radiation exposure of a subject and operator atthe time of execution of an ablation procedure.

There is available a technique called ROI fluoroscopy which is one ofthe techniques for exposure reduction. In ROI fluoroscopy, afluoroscopic image (ROI image) is generated in real time by performingfluoroscopy exclusively for the ROI required for the procedure, and theROI image is displayed as a dynamic image in real time. There isavailable a technique as an application of ROI fluoroscopy, whichcombines an ROI image generated in real time with a wide-range stillimage generated before ROI fluoroscopy, and displays the composite imageas a dynamic image in real time. ROI fluoroscopy can be used for notonly an ablation procedure but also for lower extremity and brainoperation procedures.

In procedures using ROI fluoroscopy such as ablation procedures, armsand tops are often moved and the visual field sizes are often changed.In addition, since a procedure using ROI fluoroscopy is performed for along period, the subject often moves. A positional shift therefore oftenoccurs between a still image and an ROI image. In order to eliminate thepositional shift between the still image and the ROI image, it isnecessary to update the still image. For this reason, the operatorre-captures the latest still image upon interrupting ROI fluoroscopy andswitching to the general fluoroscopy mode by, for example, changinghis/her step from one foot switch to another foot switch. As describedabove, when updating a still image, the operator operates switches, theoperator inevitably interrupts the procedure.

It is an object to provide an X-ray diagnostic apparatus which canimprove the procedure efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the arrangement of an X-ray diagnosticapparatus according to an embodiment.

FIG. 2 is a perspective view showing the outer appearance of an imagingmechanism in FIG. 1.

FIG. 3 is a view for explaining ROI setting processing by an operationunit in FIG. 1.

FIG. 4 is a view schematically showing a foot switch unit in FIG. 1.

FIG. 5 is a view showing an example of the ROI image generated by animage generation unit in FIG. 1.

FIG. 6 is a block diagram showing the arrangement of an X-ray diagnosticapparatus according to Example 1 of this embodiment.

FIG. 7 is a view schematically showing a typical procedure for automaticaperture control processing in ROI fluoroscopy according to Example 1.

FIG. 8 is a view showing an example of the composite image generated byan image combining unit in FIG. 7.

FIG. 9 is a block diagram showing the arrangement of an X-ray diagnosticapparatus according to Example 2 of this embodiment.

FIG. 10 is a view schematically showing a typical procedure forautomatic aperture control processing in ROI fluoroscopy according toExample 2.

FIG. 11 is a block diagram showing the arrangement of an X-raydiagnostic apparatus according to Example 3 of this embodiment.

FIG. 12 is a view schematically showing a typical procedure forautomatic aperture control processing in ROI fluoroscopy according toExample 3.

FIG. 13 is a view schematically showing a typical procedure forautomatic aperture control processing in ROI fluoroscopy according toExample 4.

DETAILED DESCRIPTION

In general, according to one embodiment, an X-ray diagnostic apparatusincludes an X-ray tube, an X-ray detector, an aperture-variablecollimator mechanism, an image generation unit, a combining unit, adisplay unit, a determination unit, a control unit. The X-ray tube isconfigured to generate X-rays. The X-ray detector is configured todetect X-rays generated from the X-ray tube and transmitted through asubject. The aperture-variable collimator mechanism is configured tolimit a radiation field of X-rays from the X-ray tube. The imagegeneration unit is configured to repeatedly generate a first X-ray imagebased on an output from the X-ray detector during a period in which anaperture of the collimator mechanism is a first aperture and torepeatedly generate a second X-ray image based on an output from theX-ray detector during a period in which the aperture is a secondaperture smaller than the first aperture. The combining unit isconfigured to repeatedly generate a composite image based on a latestsecond X-ray image of the repeatedly generated second X-ray images and aspecific first X-ray image of the repeatedly generated first X-rayimages for the second X-ray image is generated. The display unit isconfigured to display the repeatedly generated composite image as adynamic image in real time. The determination unit is configured todetermine whether to update the first X-ray image in the compositeimage, based on an index associated with an anatomical positional shiftbetween the first X-ray image and the second X-ray image. The controlunit is configured to enlarge the aperture of the collimator mechanismfrom the second aperture to the first aperture by controlling thecollimator mechanism when the determination unit determines to updatethe first X-ray image and to maintain the aperture of the collimatormechanism at the second aperture by controlling the collimator mechanismwhen the determination unit determines not to update the first X-rayimage.

An X-ray diagnostic apparatus according to an embodiment will bedescribed below with reference to the accompanying drawing. Note thatthis embodiment aims at an X-ray diagnostic apparatus equipped with anROI fluoroscopy technique.

The arrangement of an X-ray diagnostic apparatus according to thisembodiment will be described first with reference to FIG. 1. FIG. 1 is ablock diagram showing the arrangement of the X-ray diagnostic apparatusaccording to this embodiment. As shown in FIG. 1, the X-ray diagnosticapparatus according to the embodiment includes a system control unit 1as a main unit, an imaging mechanism 2, a C-arm driving unit 3, acollimator driving unit 4, a bed driving unit 5, a driving control unit6, an X-ray control unit 7, an image processing unit 8, a display unit9, a display control unit 10, a foot switch unit 11, an operation unit12, and a determination unit 13.

FIG. 2 is a view showing the outer appearance of the imaging mechanism2. As shown in FIGS. 2 and 1, the imaging mechanism 2 includes a C-armholder 21 rotatably mounted or fixed on a floor. The C-arm holder 21supports a C-arm 22 so as to allow it to rotate around an axis A1. Uponreceiving a driving signal from the C-arm driving unit 3, the C-armholder 21 rotates the C-arm 22 around the axis A1. The C-arm holder 21supports the C-arm 22 so as to allow it to slide around an axis A2perpendicular to the axis A1. Upon receiving a driving signal from theC-arm driving unit 3, the C-arm holder 21 causes the C-arm 22 to slidearound the axis A2 along a C shape. The C-arm driving unit 3 supplies adriving signal to the C-arm holder 21 in accordance with a controlsignal from the driving control unit 6. The intersection between theaxis A1 and the axis A2 is called an isocenter. The C-arm 22 rotatesaround the axis A1 or slides around the axis A2 while the isocenter isalways fixed spatially.

The C-arm 22 includes an X-ray tube 23 and an X-ray detector 24 facingeach other.

Upon receiving a high voltage and a filament current from a high voltagegenerator 25, the X-ray detector 24 generates X-rays. The high voltagegenerator 25 applies a high voltage or supplies a filament current inaccordance with a control signal from the X-ray control unit 7. In thefluoroscopy mode, the X-ray tube 23 continuously generates a relativelylow dose of X-rays under the control of the X-ray control unit 7. In theradiography mode, the X-ray tube 23 sporadically generates a relativelyhigh dose of X-rays as compared with the fluoroscopy mode under thecontrol of the X-ray control unit 7.

The X-ray detector 24 detects the X-rays generated from the X-ray tube23. For example, the X-ray detector 24 is implemented by a flat paneldisplay (FPD). The X-ray detector 24 includes a plurality of detectionelements arrayed two-dimensionally. Each detection element detects theX-rays generated from the X-ray tube 23, and generates an electricalsignal in accordance with the intensity of the detected X-rays. Thegenerated electrical signal is supplied to the image processing unit 8.

A collimator 26 is attached to the X-ray tube 23. The collimator 26 is amovable collimator capable of changing the size and shape of theaperture. More specifically, the collimator 26 movably supports bladesmade of a material which shields against X-rays. An aperture is definedby the blades. The blades are formed from, for example, lead. Thecollimator 26 is electrically connected to the collimator driving unit4. The collimator 26 and the collimator driving unit 4 constitute anaperture-variable collimator mechanism for limiting the radiation fieldof X-rays from the X-ray tube 23. The collimator 26 moves the bladesupon receiving a driving signal from the collimator driving unit 4. Thecollimator driving unit 4 supplies a driving signal to the collimator 26in accordance with a control signal from the driving control unit 6.Moving the blades will change the size and shape of the aperture. Whenthe collimator 26 adjusts the size and position of the aperture, thesize and position of an X-ray irradiation region onto the detectionsurface of the X-ray detector 24 are adjusted. For example, thecollimator 26 alternately switches the first and second apertures underthe control of the driving control unit 6. Assume that the secondaperture is smaller in size than the first aperture. In this case, thefirst and second apertures will be referred to as large and smallapertures respectively. As will be described later, the collimator 26adjusts the sizes and positions of the blades in accordance with whenthe operator designates or changes the size and position of an ROI withthe operation unit 12.

The image processing unit 8 generates an X-ray image associate with asubject P based on an electrical signal from the X-ray detector 24. Morespecifically, the image processing unit 8 includes an image generationunit 81, an image storage unit 83, and an image combining unit 85. Theimage generation unit 81 reads out an electrical signal from eachdetection element of the X-ray detector 24, and generates an X-ray imagebased on the readout electrical signals. The image generation unit 81repeatedly generates X-ray images for every predetermined period (e.g.,on the order of several ms) while the X-ray tube 23 generates X-rays.The generated X-ray images are supplied to the image storage unit 83,the image combining unit 85, and the display unit 9. In this case, theX-ray image generated by the image generation unit 81 during a period ofthe large aperture will be referred to as a large-aperture image, andthe X-ray image generated by the image generation unit 81 during aperiod of the small aperture will be referred to as a small-apertureimage. The image combining unit 85 generates a composite image bycombining a small-aperture image generated in real time with a specificone of repeatedly generated large-aperture images. Typically, a specificlarge-aperture image is the latest large-aperture image at this timepoint. That is, a large-aperture image is used for a still image of acomposite image. The generated composite image is supplied to thedisplay unit 9. Note that the display control unit 10 switches betweensupplying an X-ray image (large-aperture image or small-aperture image)to the display unit 9 and supplying a generated composite image to thedisplay unit 9 under the control of the system control unit 1.

The display unit 9 displays an X-ray image from the image generationunit 81 and a composite image from the image combining unit 85. As thedisplay unit 9, it is possible to use a CRT display, liquid crystaldisplay, organic EL display, plasma display, or the like, as needed.

As shown in FIGS. 1 and 2, the imaging mechanism 2 is provided with abed 27. The bed 27 includes a leg portion 28. The leg portion 28supports a top 29, on which the subject P is placed, so as to allow itto move in the horizontal and vertical directions. The leg portion 28 iselectrically connected to the bed driving unit 5. The bed driving unit 5supplies a driving signal corresponding to a control signal from thedriving control unit 6 to the leg portion 28. The bed driving unit 5 isformed from, for example, a motor such as a stepping motor. Uponreceiving a driving signal, the leg portion 28 moves the top 29 in thehorizontal or vertical direction in accordance with a driving signal.

The lower portion of the leg portion 28 is provided with the foot switchunit 11. The foot switch unit 11 includes a plurality of switches to beoperated by a foot of the operator. For example, the foot switch unit 11includes a switch for performing X-ray fluoroscopy and a switch forperforming ROI fluoroscopy. The details of the foot switch unit 11 willbe described later. The operation signal generated by the operation ofthe foot switch unit 11 is supplied to the foot switch unit 11.

The operation unit 12 accepts various kinds of commands and informationinputs from the operator via an input device, and supplies an operationsignal corresponding to a received command or information to the systemcontrol unit 1. For example, the operation unit 12 sets an ROI inaccordance with the instruction input from the operator via the inputdevice. The input device is provided separately from the foot switchunit 11. The input device which can be used include, for example, akeyboard, mouse, buttons, switches, and touch key panel.

The determination unit 13 determines whether to update thelarge-aperture image in the displayed composite image, based on an index(to be referred as to a positional shift index hereinafter) associatedwith the anatomical positional shift between the large-aperture imageand the small-aperture image. The details of the processing performed bythe determination unit 13 will be described later.

The driving control unit 6 controls the C-arm driving unit 3, thecollimator driving unit 4, and the bed driving unit 5 under the controlof the system control unit 1. For example, the driving control unit 6supplies a control signal to the C-arm driving unit 3 to move the C-arm22 to the position designated by the operator. Upon receiving thecontrol signal, the C-arm driving unit 3 supplies a driving signal tothe C-arm holder 21 to move the C-arm 22 to the position designated bythe operator. The driving control unit 6 supplies a control signal tothe bed driving unit 5 to move the top 29 to the position designated bythe operator. Upon receiving the control signal, the bed driving unit 5supplies a driving signal to the leg portion 28 to move the signal line19 to the position designated by the operator. The driving control unit6 also supplies a control signal to the collimator driving unit 4 tochange the aperture defined by the blades to the size and positiondesignated by the operator. Upon receiving the control signal, thecollimator driving unit 4 supplies a driving signal to the collimator 26to change the aperture to the size and position designated by theoperator. If, for example, the determination unit 13 determines toupdate the large-aperture image, the driving control unit 6 controls thecollimator driving unit 4 to enlarge the aperture from the smallaperture to the large aperture. If the determination unit 13 determinesnot to update the large-aperture image, the driving control unit 6controls the collimator driving unit 4 to maintain the aperture at thesmall aperture. The driving control unit 6 may control the collimatordriving unit 4 to change the aperture in accordance with an instructionfrom the foot switch unit 11 or the operation unit 12.

The system control unit 1 functions as the main unit of the X-raydiagnostic apparatus according to this embodiment and executes automaticaperture control in ROI fluoroscopy.

Automatic aperture control in ROI fluoroscopy which is performed underthe control of the system control unit 1 will be described below. Thefollowing description will exemplify an ablation procedure as a concreteexample of a clinical application of ROI fluoroscopy. An ablationprocedure is one of the cardiac treatment methods. An ablation procedureis a procedure for burning off a small amount of the living tissue of aregion as a cause of arrhythmia which is in contact with the distal endof a catheter by supplying a high-frequency current from the distal endof the catheter. An X-ray diagnostic apparatus is used for thenavigation of the catheter in an ablation procedure. Ablation procedurestend to continue for long times. In an ablation procedure, therefore, itis possible to effectively reduce the radiation dosage of the operatoror subject by using the ROI fluoroscopy mode.

Setting of an ROI by the operation unit 12 will be described first. FIG.3 is a view for explaining setting of an ROI. As shown in FIG. 3, an ROIis set on the X-ray image displayed on the display unit 9. An X-rayimage used for setting an ROI is typically a large-aperture imageobtained by X-ray radiography or fluoroscopy. In this case, alarge-diameter image is typically an X-ray image generated during aperiod in which settings are made to irradiate the entire detectionsurface of the X-ray detector 24 with X-rays.

The operator designates an image region, on the displayed large-apertureimage, which is to be observed in real time during a procedure, via aninput device such as a mouse. The designated image region is set on theROI. The positional information of the ROI is supplied to the drivingcontrol unit 6 via the system control unit 1.

When an ROI is set, the driving control unit 6 controls the collimatordriving unit 4 to change the size and position of the aperture inaccordance with the positional information of the ROI. That is, thedriving control unit 6 sets an aperture to irradiate only a local regionon the X-ray detection surface which corresponds to the ROI with X-rays.

The details of the foot switch unit 11 will be described next. FIG. 4 isa schematic view of the foot switch unit 11. As shown in FIG. 4, thefoot switch unit 11 is equipped with, for example, a radiography switch111, a fluoroscopy switch 113, and an ROI fluoroscopy switch 115.

The radiography switch 111 is a switch for switching the imaging mode tothe radiography mode. The system control unit 1 causes the X-ray controlunit 7 to execute the radiography mode while the radiography switch 111is stepped on. In this case, the X-ray control unit 7 controls the highvoltage generator 25 to generate a dose of X-rays from the X-ray tube 23in accordance with the radiography mode. Note that the dosecorresponding to the radiography mode is set to be higher than the dosecorresponding to the fluoroscopy mode.

The fluoroscopy switch 113 is a switch for switching the image capturemode to the fluoroscopy mode. The system control unit 1 causes the X-raycontrol unit 7 to execute the fluoroscopy mode while the fluoroscopyswitch 113 is stepped on. In this case, the X-ray control unit 7controls the high voltage generator 25 to generate a dose of X-rayscorresponding to the fluoroscopy mode from the X-ray tube 23. Inaddition, the system control unit 1 controls the collimator driving unit4 to set the aperture to the large aperture while the fluoroscopy switch113 is stepped on. That is, a large-diameter image is repeatedlygenerated while the fluoroscopy switch 113 is stepped on. Alarge-aperture image is used as the still image of a composite image.

The ROI fluoroscopy switch 115 is a switch for switching the imagecapture mode to the ROI fluoroscopy mode. The system control unit 1causes the X-ray control unit 7 to execute the fluoroscopy mode whilethe ROI fluoroscopy switch 115 is stepped on. In this case, the X-raycontrol unit 7 controls the high voltage generator 25 to generate a doseof X-rays corresponding to the fluoroscopy mode from the X-ray tube 23.In addition, the system control unit 1 controls the collimator drivingunit 4 to set the aperture to an aperture (small aperture) correspondingto an ROI while the ROI fluoroscopy switch 115 is stepped on. That is, asmall-aperture image is repeatedly generated while the ROI fluoroscopyswitch 115 is stepped on. Each small-aperture image generated while theaperture is set to an aperture corresponding to an ROI will be referredto as an ROI image hereinafter.

FIG. 5 is a view showing an example of an ROI image I1. As shown in FIG.5, the ROI image I1 includes an ROI region R1 and an empty region R2.The ROI region R1 corresponds to the detection surface region of theX-ray detection surface which is irradiated with X-rays. That is, theROI region R1 is the image region generated based on an electricalsignal from the detection element irradiated with X-rays. The emptyregion R2 is a portion which is shielded by the blades against X-rays,and corresponds to the detection surface region of the X-ray detectionsurface which is not irradiated with X-rays. That is, the empty regionR2 is the image region generated based on electrical signals fromdetection elements which are not irradiated with X-rays.

Note that when none of the radiography switch 11, the fluoroscopy switch113, and the ROI fluoroscopy switch 115 are stepped on, the systemcontrol unit 1 instructs the X-ray control unit 7 to stop generatingX-rays. Upon receiving the instruction to stop generating X-rays, theX-ray control unit 7 controls the high voltage generator 25 to stopgenerating X-rays from the X-ray tube 23.

Operation examples of automatic aperture control processing in ROIfluoroscopy according to this embodiment will be described separatelynext as Example 1, Example 2, and Example 3. Examples 1, 2, and 3 areclassified according to the above positional shift index used by thedetermination unit 13.

Example 1

A positional shift index according to Example 1 is the spatial positionof a C-arm 22 or top 29.

FIG. 6 is a block diagram showing the arrangement of an X-ray diagnosticapparatus according to Example 1. As shown in FIG. 6, the X-raydiagnostic apparatus according to Example 1 includes a positionrecording unit 14 in addition to the units of the X-ray diagnosticapparatus according to this embodiment.

The position recording unit 14 records the positional information of theC-arm 22 and the positional information of the top 29. Morespecifically, a driving control unit 6 transmits, to the positionrecording unit 14, the positional information of the C-arm 22 every timethe spatial position of the C-arm 22 is changed, and the positionalinformation of the top 29 every time the spatial position of the top 29is changed. The position recording unit 14 receives the positionalinformation of the C-arm 22 and the positional information of the top 29from the driving control unit 6, and records the pieces of receivedpositional information on an internal memory or the like. The positionalinformation of the C-arm 22 is information concerning the position(spatial position) of the C-arm 22 in a real space. More specifically,the positional information of the C-arm 22 includes informationconcerning the rotational angles of the C-arm 22 around an axis A1 andan axis A2. Note that the positional information of the C-arm 22 is notlimited to this information. If the C-arm 22 includes a movable axis inaddition to the axes A1 and A2, the positional information of the C-arm22 may include information concerning the spatial position defined bythese movable axes. The positional information of the top 29 isinformation concerning the position (spatial position) of the top 29 ina real space. More specifically, the positional information of the top29 includes the spatial positions of the top 29 in the vertical andhorizontal directions. Assume that the position recording unit 14associates the time and aperture size (large or small aperture) witheach piece of positional information of the C-arm 22 or each piece ofpositional information of the top 29.

A determination unit 13 determines, every time an ROI image isgenerated, whether to update a still image, based on a preset thresholdand the difference in spatial position between a still image(large-aperture image) when it is generated and an ROI image(small-aperture image) when it is generated. If the determination unit13 determines to update the still image, the driving control unit 6enlarges the aperture from the small aperture to the large aperture. Ifthe determination unit 13 determines not to update the still image, thedriving control unit 6 maintains the aperture at the small aperture.

An example of automatic aperture control processing in ROI fluoroscopyaccording to Example 1 will be described below with reference to FIG. 7.FIG. 7 is a view schematically showing a typical procedure for automaticaperture control processing in ROI fluoroscopy according to Example 1.

First of all, the operator steps on the fluoroscopy switch at time t1 toperform X-ray fluoroscopy under the control of the driving control unit6. As described above, in the fluoroscopy mode, the aperture is set tothe large aperture, an image generation unit 81 repeatedly generates alarge-aperture image in real time, and a display unit 9 displays alarge-aperture image IS as a dynamic image in real time. Thislarge-aperture image is typically generated to be used for the stillimage of a composite image to be generated afterward. The operatorobserves the large-aperture image displayed on the display unit 9 anddetermines whether a large-aperture image suitable for a still image hasbeen generated. In addition, the operator may set an ROI on alarge-aperture image via an operation unit 12. Note that the operatormay set an ROI before time t1.

Upon determining that a large-aperture image suitable for a still imagehas been generated, the operator steps off the fluoroscopy switch andsteps on the ROI fluoroscopy switch (time t2). The large-aperture image(LHI: last holding image) displayed on the display unit 9 at the timewhen the operator steps on the ROI fluoroscopy switch is displayed as astill image on the display unit 9. An image storage unit 83 stores thisstill image. When the operator steps on the ROI fluoroscopy switch, thedriving control unit 6 starts ROI fluoroscopy. More specifically, thedriving control unit 6 controls a collimator driving unit 4 to reducethe aperture of a collimator 26 from the large aperture to the smallaperture. An X-ray control unit 7 also controls a high voltage generator25 to continuously generate X-rays having a dose for fluoroscopy from anX-ray tube 23. Irradiating a subject P with X-rays upon limiting theirradiation to the small aperture corresponding to the ROI can reducethe radiation dosage of the subject P or the like. During the ROIfluoroscopy mode, the image generation unit 81 repeatedly generates anROI image IR in real time. Every time the ROI image IR is generated, animage combining unit 85 repeatedly generates a composite image IC basedon the ROI image IR and the large-aperture image IS stored as a stillimage in the image storage unit 83. The display unit 9 displays thegenerated composite image IC as a dynamic image in real time.

FIG. 8 is a view showing an example of a composite image I2. As shown inFIG. 8, the composite image I2 includes an ROI image region R3 and astill image region R4. The ROI image region R3 corresponds to the ROIregion in the ROI image generated in real time. The still image regionR4 corresponds to an image region other than the ROI region of the stillimage. That is, the ROI image region R3 in the composite image I2 isdisplayed as a dynamic image, and the still image region R4 is displayedas a still image. Combining and displaying an ROI image and a stillimage in this manner can display only a target ROI as a dynamic image inreal time while allowing the operator to easily comprehend the positionof the ROI in a still image. It is therefore possible to reduce theradiation dosage more than in the fluoroscopy mode while maintaining theoperability in the fluoroscopy mode.

For example, the image combining unit 85 combines an ROI region in anROI image and an image region other than an ROI region in a still imageby using a superimposing technique. This generates a composite imagelike that obtained by pasting an ROI region cut from an ROI image to astill image.

According to the above description, the apparatus performs fluoroscopyonly with the small aperture at the time of ROI fluoroscopy. However,this embodiment is not limited to this. For example, the apparatus mayperform fluoroscopy with the large aperture for only a preset time atthe start of ROI fluoroscopy. The preset time is preferably set to arelatively short period, e.g., 1 sec. The display unit 9 displays thelarge-aperture image generated at the start of ROI fluoroscopy. When thepreset time has elapsed, the driving control unit 6 switches the largeaperture to the small aperture, and the X-ray control unit 7 performsfluoroscopy with the small aperture, as described above.

In some case, the apparatus moves the C-arm 22 or the top 29 inaccordance with an instruction issued by the operator via the operationunit 12 during the execution of the ROI fluoroscopy mode. Since a stillimage is an image generated in the past, when the C-arm 22 or top 29moves, an anatomical positional shift occurs between an ROI image and astill image. Even if the operator observes the composite image based onthe ROI image and the still image between which the positional shift hasoccurred, the operator cannot properly determine the position of acatheter or the like. Displaying the composite image based on the ROIimage and the still image between which the positional shift hasoccurred will degrade operability.

The X-ray diagnostic apparatus according to this embodiment eliminatesthe above problem accompanying the movement of the C-arm 22 or top 29 byusing positional information concerning the spatial position of theC-arm 22 or top 29. For this purpose, the position recording unit 14records the spatial position of the C-arm 22 and the spatial position ofthe top 29 during an ablation procedure. Recording timings may be set asfollows. The position recording unit 14 may record the spatial positionof the C-arm 22 and the spatial position of the top 29 at predeterminedtime intervals or every time the C-arm 22 or the top 29 is moved. Theposition recording unit 14 records each spatial position in associationwith the time and the identifier of an aperture size (large or smallaperture).

The determination unit 13 determines whether to update the still image,by using positional information concerning the spatial position recordedon the position recording unit 14. The determination unit 13individually determines a change in the spatial position of the C-arm 22and a change in the spatial position of the top 29. Note that thedetermination unit 13 may perform the determination processing atpredetermined time intervals or every time the C-arm 22 or the top 29 ismoved.

In the case of the C-arm 22, the determination unit 13 performs thedetermination in the following manner. In the determination processingassociated with the C-arm 22, the determination unit 13 reads out thereal-time spatial position of the C-arm 22 and the spatial position ofthe C-arm 22 at a reference time from the position recording unit 14.The spatial position at the reference time is set to the spatialposition of the C-arm 22 at the time of generation of a large-apertureimage. For example, the spatial position at the reference time is set tothe spatial position of the C-arm 22 at the time point when the ROIfluoroscopy switch is stepped on. If the ROI fluoroscopy switch has beenstepped on a plurality of number of times, the spatial position of theC-arm 22 at the latest time when the switch was stepped on is set to thespatial position at the reference time. Upon reading out the spatialpositions, the determination unit 13 calculates the difference betweenthe real-time spatial position of the C-arm 22 and the spatial positionof the C-arm 22 at the reference time. The determination unit 13 thendetermines whether the calculated difference exceeds a threshold for theC-arm 22. The operator can arbitrarily set a threshold via the operationunit 12. For example, it is preferable to set a threshold to the maximumvalue or the like of the allowable range of positional shifts for theoperator. If no positional shift is allowed between an ROI image and astill image, the threshold is preferably set to 0.

In the case of the top 29, the determination unit 13 performs thedetermination in the same manner as in the case of the C-arm 22. In thedetermination processing associated with the top 29, the determinationunit 13 reads out the real-time spatial position of the top 29 and thespatial position of the top 29 at a reference time from the positionrecording unit 14. The spatial position at the reference time is set tothe spatial position of the top 29 at the time of generation of alarge-aperture image. For example, the spatial position at the referencetime is set to the spatial position of the top 29 at the time point whenthe ROI fluoroscopy switch is stepped on. If the ROI fluoroscopy switchwas stepped on a plurality of number of times, the spatial position ofthe top 29 at the latest time when the switch was stepped on is set tothe spatial position at the reference time. Upon reading out the spatialpositions, the determination unit 13 calculates the difference betweenthe real-time spatial position of the top 29 and the spatial position ofthe top 29 at the reference time. The determination unit 13 thendetermines whether the calculated difference exceeds a threshold for thetop 29. The operator can arbitrarily set a threshold via the operationunit 12. Note that thresholds are individually set for the C-arm 22 andthe top 29.

Upon determining that the difference does not exceed the threshold, thedetermination unit 13 determines that there is no need to update thestill image. More specifically, upon determining that both thedifferences associated with the C-arm 22 and the top 29 do not exceedthe thresholds, the determination unit 13 determines that there is noneed to update the still image. In this case, the driving control unit 6controls the collimator driving unit 4 to maintain the aperture at thesmall aperture. The X-ray control unit 7 controls the high voltagegenerator 25 to continuously generate X-rays for fluoroscopy.

Upon determining that the differences exceed the thresholds (time t3),the determination unit 13 determines to update the still image. Morespecifically, upon determining that at least one of the differencesassociated with the C-arm 22 and the top 29 does not exceed thecorresponding threshold, the determination unit 13 determines that thereis no need to update the still image. In this case, the driving controlunit 6 controls the collimator driving unit 4 to automatically enlargethe aperture from the small aperture to the large aperture. The X-raycontrol unit 7 controls the high voltage generator 25 to repeatedlygenerate X-rays for fluoroscopy from the X-ray tube 23. With thisoperation, the image generation unit 81 generates the large-apertureimage IS. Since the new large-aperture image IS used to update the stillimage, at least one large-aperture image may be generated. Note thatswitching from the large aperture to the small aperture in accordancewith the determination result obtained by the determination unit 13 isperformed without making the operator change his/her step from the ROIfluoroscopy switch to the fluoroscopy switch. That is, the apparatusswitches from the small aperture to the large aperture, while the ROIfluoroscopy switch is kept stepped on, to generate the newlarge-aperture image IS. The image storage unit 83 stores the generatednew large-aperture image IS as a new still image.

After the lapse of a predetermined period since the aperture wasenlarged to the large aperture (time t4), the driving control unit 6automatically switches to the ROI fluoroscopy mode. That is, the drivingcontrol unit 6 controls the collimator driving unit 4 to automaticallyreduce the aperture from the large aperture to the small aperture. TheX-ray control unit 7 controls the high voltage generator 25 torepeatedly generate X-rays for fluoroscopy. Note that it is preferableto set this predetermined period to a period during which at least onelarge-aperture image can be generated. During the ROI fluoroscopy mode,the image generation unit 81 repeatedly generates an ROI image IR inreal time. Every time the ROI image IR is generated, the image combiningunit 85 repeatedly generates the composite image IC based on the ROIimage IR and the large-aperture image IS stored as a new still image inthe image storage unit 83 in real time. The display unit 9 displays thegenerated composite image IC as a dynamic image in real time.

According to the above description, the aperture is automaticallyswitched from the large aperture to the small aperture after the lapseof the predetermined period. However, Example 1 is not limited to this.For example, the aperture may be switched from the large aperture to thesmall aperture when the operator re-steps on the ROI fluoroscopy switch.This makes it possible to set, as a still image on a composite image,the large-aperture image determined as an image suitable for a stillimage by the operator.

This is the end of the description of an operation example according toExample 1.

As described above, the X-ray diagnostic apparatus according to Example1 automatically switches the aperture from the small aperture to thelarge aperture when the positional shift amount between the spatialposition of the C-arm 22 or top 29 at the time of generation of a stillimage and the real-time spatial position of the C-arm 22 or top 29exceeds a threshold. In the X-ray diagnostic apparatus according toExample 1, this can automatically update a still image when ananatomical positional shift occurs between the still image and an ROIimage, thereby providing a composite image with a small anatomicalpositional shift amount. It is therefore not necessary for the operatorto change his/her step from one switch to another switch at this time.The operator can therefore update a still image without giving anythought to changing his/her step from one switch to another switch andconcentrate on the ablation procedure.

Example 2

A positional shift index according to Example 2 is a real-time X-raygeneration duration time in the ROI fluoroscopy mode.

FIG. 9 is a block diagram showing the arrangement of an X-ray diagnosticapparatus according to Example 2. As shown in FIG. 9, the X-raydiagnostic apparatus according to Example 2 includes an X-ray generationduration time measurement unit 15 in addition to the units of the X-raydiagnostic apparatus according to this embodiment.

The X-ray generation duration time measurement unit 15 repeatedlymeasures the time (to be referred to as an X-ray generation durationtime hereinafter) during which X-rays are continuously generated fromthe start time of the ROI fluoroscopy mode. If the X-ray generationduration time is relatively long, it is expected that a subject P willmove. In other words, if the X-ray generation duration time isrelatively long, it can be estimated that an anatomical positional shifthas occurred between an ROI image and a still image. The measured X-raygeneration duration time is supplied to a determination unit 13.

The determination unit 13 determines whether to update a still image,based on the X-ray generation duration time, every time an ROI image isgenerated. If the determination unit 13 determines to update the stillimage, a driving control unit 6 enlarges the aperture from the smallaperture to the large aperture. If the determination unit 13 determinesnot to update the still image, the driving control unit 6 maintains theaperture at the small aperture.

An example of automatic aperture control processing in ROI fluoroscopyaccording to Example 2 will be described below with reference to FIG.10. FIG. 10 is a view schematically showing a typical procedure forautomatic aperture processing in ROI fluoroscopy according to Example 2.The same processing contents as automatic aperture control processing inROI fluoroscopy according to Example 1 will be briefly described.

First of all, at time t1, the operator steps on the fluoroscopy switch,and the apparatus performs X-ray fluoroscopy under the control of thedriving control unit 6. As described above, in the fluoroscopy mode, theaperture is set to the large aperture, and an image generation unit 81repeatedly generates a large-aperture image in real time. A display unit9 displays a large-aperture image IS as a dynamic image in real time.

Upon determining that a large-aperture image suitable for a still imageis generated, the operator steps off the fluoroscopy switch and steps onthe ROI fluoroscopy switch (time t2). The large-aperture image displayedon the display unit 9 when the operator steps on the ROI fluoroscopyswitch is displayed as a still image on the display unit 9. When theoperator steps on the ROI fluoroscopy switch, the driving control unit 6starts ROI fluoroscopy. During the ROI fluoroscopy mode, the imagegeneration unit 81 repeatedly generates an ROI image IR in real time.Every time the ROI image IR is generated, an image combining unit 85repeatedly generates a composite image IC based on the ROI image IR andthe large-aperture image IS stored as a still image in an image storageunit 83 in real time. The display unit 9 displays the generatedcomposite image IC as a dynamic image in real time.

When the operator steps on the ROI fluoroscopy switch (time t2), theX-ray generation duration time measurement unit 15 repeatedly measuresan X-ray generation duration time. The determination unit 13 determines,in the ROI fluoroscopy mode, whether the measured X-ray generationduration time exceeds a predetermined time set in advance. The operatorcan arbitrarily set the predetermined time town arbitrary value via theoperation unit 12.

Upon determining that the X-ray generation duration time does not exceedthe predetermined time, the determination unit 13 determines that thereis no need to update the still image. In this case, the driving controlunit 6 controls the collimator driving unit 4 to maintain the apertureat the small aperture. An X-ray control unit 7 controls a high voltagegenerator 25 to make it continuously generate X-rays for fluoroscopy.

Upon determining that the X-ray generation duration time has exceededthe predetermined time (time t3), the determination unit 13 determinesto update the still image. In this case, the driving control unit 6controls the collimator driving unit 4 to automatically enlarge theaperture from the small aperture to the large aperture. The X-raycontrol unit 7 controls the high voltage generator 25 to make an X-raytube 23 repeatedly generate X-rays for fluoroscopy. With this operation,the image generation unit 81 generates the large-aperture image IS.

After the lapse of a predetermined period since the aperture wasenlarged to the large aperture (time t4), the driving control unit 6automatically switches to the ROI fluoroscopy mode. That is, the drivingcontrol unit 6 controls the collimator driving unit 4 to automaticallyreduce the aperture from the large aperture to the small aperture. TheX-ray control unit 7 controls the high voltage generator 25 torepeatedly generate X-rays for fluoroscopy. During the ROI fluoroscopymode, the image generation unit 81 repeatedly generates the ROI image IRin real time. Every time the ROI image IR is generated, the imagecombining unit 85 repeatedly generates the composite image IC based onthe ROI image IR and the large-aperture image IS stored as a new stillimage in the image storage unit 83 in real time. The display unit 9displays the generated composite image IC as a dynamic image in realtime.

This is the end of the description of an operation example according toExample 2.

As described above, the X-ray diagnostic apparatus according to Example2 automatically switches the aperture from the small aperture to thelarge aperture when the X-ray generation duration time from the start ofROI fluoroscopy mode exceeds a predetermined period. This allows theX-ray diagnostic apparatus according to Example 2 to automaticallyupdate a still image when it is estimated that an anatomical positionalshift will occur between the still image and an ROI image, therebyproviding a composite image with a small anatomical positional shiftamount. It is therefore not necessary for the operator to change his/herstep from one switch to another switch at this time. The operator cantherefore update a still image without giving any thought to changinghis/her step from one switch to another switch and concentrate on theablation procedure.

Example 3

A positional shift index according to Example 3 is a real-time X-raynon-generation duration time in the ROI fluoroscopy mode.

FIG. 11 is a block diagram showing the arrangement of an X-raydiagnostic apparatus according to Example 3. As shown in FIG. 11, theX-ray diagnostic apparatus according to Example 3 includes an X-raynon-generation duration time measurement unit 16 in addition to theunits of the X-ray diagnostic apparatus according to this embodiment.

The X-ray non-generation duration time measurement unit 16 repeatedlymeasures the time (to be referred to as the X-ray non-generationduration time hereinafter) during which no X-rays are continuouslygenerated from the point of time when the generation of X-rays in theROI fluoroscopy mode is stopped. If the X-ray non-generation durationtime is relatively long, it is expected that a subject P has moved or aC-arm 22 or top 29 has been moved. In other words, if the X-raynon-generation duration time is relatively long, it can be estimatedthat an anatomical positional shift has occurred between the ROI imageand the still image. The measured X-ray non-generation duration time issupplied to a determination unit 13.

The determination unit 13 determines, based on the X-ray non-generationduration time, whether to update the still image. If the determinationunit 13 determines to update the still image, a driving control unit 6enlarges the aperture from the small aperture to the large aperture. Ifthe determination unit 13 determines not to update the still image, thedriving control unit 6 maintains the aperture at the small aperture.

An example of automatic aperture control processing in ROI fluoroscopyaccording to Example 3 will be described below with reference to FIG.12. FIG. 12 is a view schematically showing a typical procedure forautomatic aperture control processing in ROI fluoroscopy according toExample 3. The same processing contents as automatic aperture controlprocessing in ROI fluoroscopy according to Example 1 will be brieflydescribed.

First of all, at time t1, the operator steps on the fluoroscopy switch,and the apparatus performs X-ray fluoroscopy under the control of thedriving control unit 6. As described above, in the fluoroscopy mode, theaperture is set to the large aperture, and an image generation unit 81repeatedly generates a large-aperture image in real time. A display unit9 displays a large-aperture image IS as a dynamic image in real time.

Upon determining that a large-aperture image suitable for a still imageis generated, the operator steps off the fluoroscopy switch and steps onthe ROI fluoroscopy switch (time t2). The large-aperture image displayedon the display unit 9 when the operator steps on the ROI fluoroscopyswitch is displayed as a still image on the display unit 9. When theoperator steps on the ROI fluoroscopy switch, the driving control unit 6starts ROI fluoroscopy. During the ROI fluoroscopy mode, the imagegeneration unit 81 repeatedly generates an ROI image IR in real time.Every time the ROI image IR is generated, an image combining unit 85repeatedly generates a composite image IC based on the ROI image IR andthe large-aperture image IS stored as a still image in an image storageunit 83 in real time. The display unit 9 displays the generatedcomposite image IC as a dynamic image in real time.

The operator sometimes stops generating X-rays at the time of ROIfluoroscopy. When, for example, the operator steps off all the switchesof a foot switch unit 11, an X-ray control unit 7 controls a highvoltage generator 25 to stop generating X-rays from an X-ray tube 23.That is, the apparatus stops ROI fluoroscopy. When the apparatus stopsthe ROI fluoroscopy (time t2′), the X-ray non-generation duration timemeasurement unit 16 repeatedly measures an X-ray non-generation durationtime. The determination unit 13 repeatedly determines whether themeasured X-ray non-generation duration time has exceeded a predeterminedtime set in advance during an X-ray stop period. The operator canarbitrarily set the predetermined time to an arbitrary value via theoperation unit 12.

Upon determining that the X-ray non-generation duration time does notexceed the predetermined time, the determination unit 13 determines thatthere is no need to update the still image. In this case, the drivingcontrol unit 6 controls the collimator driving unit 4 to maintain theaperture at the small aperture. An X-ray control unit 7 controls a highvoltage generator 25 to make it continuously generate X-rays forfluoroscopy.

Upon determining that the X-ray non-generation duration time hasexceeded the predetermined time (time t3), the determination unit 13determines to update the still image. In this case, the driving controlunit 6 controls the collimator driving unit 4 to automatically enlargethe aperture from the small aperture to the large aperture. The X-raycontrol unit 7 controls the high voltage generator 25 to repeatedlygenerate X-rays for fluoroscopy from an X-ray tube 23. With thisoperation, the image generation unit 81 generates the large-apertureimage IS.

After the lapse of a predetermined period since the aperture wasenlarged to the large aperture (time t4), the driving control unit 6automatically switches to the ROI fluoroscopy mode. That is, the drivingcontrol unit 6 controls the collimator driving unit 4 to automaticallyreduce the aperture from the large aperture to the small aperture. TheX-ray control unit 7 controls the high voltage generator 25 torepeatedly generate X-rays for fluoroscopy. During the ROI fluoroscopymode, the image generation unit 81 repeatedly generates the ROI image IRin real time. Every time the ROI image IR is generated, the imagecombining unit 85 repeatedly generates the composite image IC based onthe ROI image IR and the large-aperture image IS stored as a new stillimage in the image storage unit 83 in real time. The display unit 9displays the generated composite image IC as a dynamic image in realtime.

This is the end of the description of an operation example according toExample 3.

As described above, the X-ray diagnostic apparatus according to Example3 automatically switches the aperture from the small aperture to thelarge aperture when the X-ray non-generation duration time from when thegeneration of X-rays is stopped exceeds a predetermined period. Thisallows the X-ray diagnostic apparatus according to Example 3 toautomatically update a still image when an anatomical positional shiftoccurs between the still image and an ROI image, thereby providing acomposite image with a small anatomical positional shift amount. It istherefore not necessary for the operator to change his/her step from oneswitch to another switch at this time. The operator can therefore updatea still image without giving any thought to changing his/her step fromone switch to another switch and concentrate on the ablation procedure.

Example 4

A positional shift index according to Example 4 is informationassociated with the ON state of the ROI fluoroscopy mode and informationassociated with the OFF state of the mode.

The arrangement of the X-ray diagnostic apparatus according to thefourth embodiment is the same as that shown in FIG. 1.

As described above, a foot switch unit 11 is equipped with an ROIfluoroscopy switch. While the ROI fluoroscopy switch is stepped on, ROIfluoroscopy is set ON. While the ROI fluoroscopy switch is not steppedon, ROI fluoroscopy is set OFF. That is, when the operator steps on theROI fluoroscopy switch, ROI fluoroscopy is switched ON. When theoperator steps on the ROI fluoroscopy switch, the foot switch unit 11supplies an ON signal to a driving control unit 6 via a system controlunit 1. Upon receiving the ON signal, the driving control unit 6executes ROI fluoroscopy in the above manner. When the operator stepsoff the ROI fluoroscopy switch, the foot switch unit 11 supplies an OFFsignal to the driving control unit 6 via the system control unit 1. Uponreceiving the OFF signal, the driving control unit 6 interrupts ROIfluoroscopy, as described above. The ON and OFF signals are supplied toa determination unit 13 via the system control unit 1.

The determination unit 13 determines whether to update a still image inaccordance with switching of the ROI fluoroscopy mode. Morespecifically, the determination unit 13 determines to update a stillimage, every time the apparatus switches to the ROI fluoroscopy mode. Inother cases, the determination unit 13 determines not to update a stillimage. If the determination unit 13 determines to update a still image,the driving control unit 6 enlarges the aperture from the small apertureto the large aperture. In contrast, if the determination unit 13determines not to update the still image, the driving control unit 6maintains the aperture at the small aperture.

An example of automatic aperture control processing in ROI fluoroscopyaccording to Example 4 will be described below with reference to FIG.13. FIG. 13 is a view schematically showing a typical procedure forautomatic aperture control processing in ROI fluoroscopy according toExample 4. The same processing contents as automatic aperture controlprocessing in ROI fluoroscopy according to Example 1 will be brieflydescribed.

First of all, at time t1, the operator steps on the fluoroscopy switch,and the apparatus performs X-ray fluoroscopy under the control of thedriving control unit 6. As described above, in the fluoroscopy mode, theaperture is set to the large aperture, and an image generation unit 81repeatedly generates a large-aperture image in real time. A display unit9 displays a large-aperture image IS as a dynamic image in real time.Since the ROI fluoroscopy switch is not stepped on while the fluoroscopyswitch is stepped on, the foot switch unit 11 supplies an OFF signal tothe determination unit 13. Upon receiving the OFF signal, thedetermination unit 13 determines not to update the still image.

Upon determining that a large-aperture image suitable for a still imageis generated, the operator steps off the fluoroscopy switch and steps onthe ROI fluoroscopy switch (time t2). The large-aperture image displayedon the display unit 9 when the operator steps on the ROI fluoroscopyswitch is displayed as a still image on the display unit 9. When theoperator steps on the ROI fluoroscopy switch, the driving control unit 6starts ROI fluoroscopy. When the apparatus switches to ROI fluoroscopy,the driving control unit 6 controls a collimator driving unit 4 toautomatically reduce the aperture from the large aperture to the smallaperture. An X-ray control unit 7 controls a high voltage generator 25to repeatedly generate X-rays for fluoroscopy from an X-ray tube 23.With this operation, during the ROI fluoroscopy mode, the imagegeneration unit 81 repeatedly generates an ROI image IR in real time.Every time the ROI image IR is generated, an image combining unit 85repeatedly generates a composite image IC based on the ROI image IR andthe large-aperture image IS stored as a still image in an image storageunit 83 in real time. The display unit 9 displays the generatedcomposite image IC as a dynamic image in real time.

As described above, in some cases, a subject P moves or a C-arm 22 or atop 29 moves at the time of ROI fluoroscopy. In such a case, ananatomical positional shift occurs between an ROI image and a stillimage. In this case, the operator steps on the ROI fluoroscopy switchagain (t3). When the operator re-steps on the ROI fluoroscopy switch,the foot switch unit 11 supplies an ON signal to the determination unit13. Upon receiving the ON signal, the determination unit 13 determinesto update the still image. In this case, first of all, the drivingcontrol unit 6 controls the collimator driving unit 4 to automaticallyenlarge the aperture from the small aperture to the large aperture. TheX-ray control unit 7 controls the high voltage generator 25 torepeatedly generate X-rays for fluoroscopy from the X-ray tube 23. Withthis operation, the image generation unit 81 generates thelarge-aperture image IS.

After the lapse of a predetermined period since the aperture wasenlarged to the large aperture (time t4), the driving control unit 6automatically switches to the ROI fluoroscopy mode. This predeterminedperiod is set to the time during which the large-aperture image IScorresponding to at least one frame can be generated. That is, thedriving control unit 6 controls the collimator driving unit 4 toautomatically reduce the aperture from the large aperture to the smallaperture. The X-ray control unit 7 controls the high voltage generator25 to repeatedly generate X-rays for fluoroscopy. During the ROIfluoroscopy mode, the image generation unit 81 repeatedly generates theROI image IR in real time. Every time the ROI image IR is generated, theimage combining unit 85 repeatedly generates the composite image ICbased on the ROI image IR and the large-aperture image IS stored as anew still image in the image storage unit 83 in real time. The displayunit 9 displays the generated composite image IC as a dynamic image inreal time.

This is the end of the description of an operation example according toExample 4.

As described above, the X-ray diagnostic apparatus according to Example4 automatically updates a still image when the operator steps on the ROIfluoroscopy switch. When it is estimated that an anatomical positionalshift will occur between a still image and an ROI image, the apparatuscan automatically update the still image and can provide a compositeimage with a little anatomical positional shift amount in the X-raydiagnostic apparatus according to Example 4.

[Effects]

As has been described above, this embodiment can provide an X-raydiagnostic apparatus which can improve procedure efficiency.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An X-ray diagnostic apparatus comprising: an X-ray tube configured to generate X-rays; an X-ray detector configured to detect X-rays generated from the X-ray tube and transmitted through a subject; an aperture-variable collimator mechanism configured to limit a radiation field of X-rays from the X-ray tube; an image generation unit configured to repeatedly generate a first X-ray image based on an output from the X-ray detector during a period in which an aperture of the collimator mechanism is a first aperture and to repeatedly generate a second X-ray image based on an output from the X-ray detector during a period in which the aperture is a second aperture smaller than the first aperture; a combining unit configured to repeatedly generate a composite image based on a latest second X-ray image of the repeatedly generated second X-ray images and a specific first X-ray image of the repeatedly generated first X-ray images for the second X-ray image is generated; a display unit configured to display the repeatedly generated composite image as a dynamic image in real time; a determination unit configured to determine whether to update the first X-ray image in the composite image, based on an index associated with an anatomical positional shift between the first X-ray image and the second X-ray image; and a control unit configured to enlarge the aperture of the collimator mechanism from the second aperture to the first aperture by controlling the collimator mechanism when the determination unit determines to update the first X-ray image and to maintain the aperture of the collimator mechanism at the second aperture by controlling the collimator mechanism when the determination unit determines not to update the first X-ray image.
 2. The X-ray diagnostic apparatus of claim 1, further comprising: a top on which the subject is placed; and a top support mechanism configured to support to move the top, wherein the index is a spatial position of the top, and the determination unit determines whether to update the first X-ray image, based on a threshold and a difference between a first spatial position at the time of generation of the first X-ray image and a second spatial position at the time of generation of the second X-ray image.
 3. The X-ray diagnostic apparatus of claim 1, wherein the index is a generation duration time of X-rays from the X-ray tube in real time, and the determination unit determines whether to update the first X-ray image, based on the generation duration time and a threshold.
 4. The X-ray diagnostic apparatus of claim 1, wherein the index is a non-generation duration time of X-rays from the X-ray tube in real time, and the determination unit determines whether to update the first X-ray image, based on the non-generation duration time and a threshold.
 5. The X-ray diagnostic apparatus of claim 1, further comprising an input unit configured to switch between an ON state and OFF state of an ROI fluoroscopy mode, wherein the index is information associated with the ON state of the ROI fluoroscopy mode and information associated with the OFF state, and the determination unit determines to update the first X-ray image for the ROI fluoroscopy mode is switched to the ON state via the input unit.
 6. The X-ray diagnostic apparatus of claim 1, further comprising an arm on which the X-ray tube and the X-ray detector are pivotally mounted, the index is a spatial position of the arm, and the determination unit determines whether to update the first X-ray image, based on a threshold and a difference between a first spatial position at the time of generation of the first X-ray image and a second spatial position at the time of generation of the second X-ray image.
 7. The X-ray diagnostic apparatus of claim 1, wherein the specific first X-ray image is the latest X-ray image of the repeatedly generated first X-ray images. 